1. Field of the Invention
The invention relates to a method and a device for measuring the phase noise of a signal.
2. Related Technology
According to the prior art, the phase noise of a signal source, for example, a frequency oscillator, is measured with a device corresponding to FIG. 1.
For this purpose, the frequency signal generated by the signal source 1 with a basic frequency f1 and a phase noise f1(t) superimposed on this basic frequency f1 is mixed in a mixer 2 with the reference-frequency signal generated by a reference-signal source 3 with a basic frequency f2. A signal component with a frequency, which corresponds to the addition of the frequencies of the frequency signal generated by the signal source 1 and the frequency signal generated by the reference-signal source 3, and a signal component with a frequency, which corresponds to the subtraction of the frequencies of the frequency signal generated by the signal source 1 and of the frequency signal generated by the reference-signal source 3 are disposed at the output of the mixer 2. In a low-pass filter 4 connected downstream of the mixer 2, the signal component with the additive combination of frequencies of the signal source 1 and the reference-signal source 3 is screened out. The signal component of the subtracted combination of the frequencies of the signal source 1 and of the reference-signal source 3 is supplied as the control difference of a phase-locked loop to the controlled reference-signal source 3 for minimization. In the settled condition of the phase-locked loop, the basic frequency f2 of the reference-frequency signal corresponds to the basic frequency f1 of the frequency signal, so that a signal with a frequency corresponding to the phase noise f1(t) is disposed at the output of the low-pass filter 4
The disadvantage with a device of this kind for measuring the phase noise of a signal is the fact that the reference-frequency signal generated by the reference-signal source 3 is also contaminated with a phase noise f2(t), which is undesirably superimposed on the phase noise f1(t) of the signal source 1 in the signal at the output of the low-pass filter 4.
In order to overcome this significant disadvantage, the specification US 2005/0238094 A1 discloses a method and a device for measuring phase noise, which are based on a correlation analysis. The measurement signal V generated by a signal source 100 with a given frequency f1 to be measured and a superimposed phase noise f1(t) is subdivided in a downstream distributor 105 into a first measurement signal V1′ in a first signal path and a second measurement signal V2′ in a second signal path. The first or respectively second measurement signal V1′ or V2′ is converted in each case in a mixer 180 or 190 into a first or respectively second signal V1 or V2 in each case with reduced frequency. The mixer frequency of the first and second mixer 180 and 190 are generated in each case by a local oscillator 181 and 191.
The first signal V1 is supplied to a first phase-locked loop 110, consisting of a first mixer 111, a subsequent, first low-pass filter 112 and a controlled, first reference-signal source 113 corresponding to the first phase-locked loop in FIG. 1, in order to generate a third signal V3 with a third frequency f3(t) compensated by the measurement frequency f1 relative to the first frequency (f1+f1(t))/N of the first signal V1. The third frequency f3(t) contains the phase noise f1(t) of the signal source 100 and the phase noise f21(t) of the controlled, first reference-signal source 113. The second signal V2 is supplied in a similar manner to a second phase-locked loop 120, consisting of a second mixer 121, a subsequent, second low-pass filter 122 and a controlled, second reference-signal source 123 in order to generate a fourth signal V4 with a fourth frequency f4(t) compensated by the measurement frequency f1 relative to the second frequency (f1+f1(t))/M of the second signal V2. The fourth frequency f4(t) contains the phase noise f1(t) of the signal source 100 and the phase noise f22(t) of the controlled, second reference-signal source 123.
After an analog/digital conversion of the third signal V3 in a first analog/digital converter 150 and, based upon the latter, a Fourier analysis of the digitized third signal V3 in a first Fast-Fourier transformer 151 (FFT), and in a corresponding manner, an analog/digital conversion of the fourth signal V4 in a second analog/digital converter 160 and, based upon the latter, a Fourier analysis of the digitized fourth signal V4 in a second Fast-Fourier transformer 161, the associated correlation spectrum is calculated from the two Fourier spectra of the third and fourth signals V3 and V4 in a downstream correlator 170.
An averaging unit 171 disposed downstream of the correlator 140 implements an averaging over time of the correlation spectra calculated in the correlator 170 over a sufficiently long time interval T. The averaging result contains only the spectral components of the phase noise f1(t) contained in the third and fourth signal V3 and V4 of the signal source 100, but not the spectral components of the phase noise f21(t) and f22(t) of the controlled first and second reference-signal source 113 and 123 also contained in the third and fourth signal V3 and V4 and not correlated with one another.
In this context, the use of a mixer for frequency reduction in order to achieve a frequency expansion of the phase noise f1(t) to be measured within the limited frequency-measurement range of the correlator represents a comparatively cost-intensive realization.